The present invention relates to a method and apparatus for detecting decrease in tire air-pressure and a program for judging decompression of a tire.
An apparatus for detecting decrease in tire air-pressure in which decompression of a tire is detected on the basis of rotational (wheel speed) information of four wheel tires mounted to a vehicle is conventionally known. Such an apparatus employs a theory that a rotational angular velocity of a decompressed tire is increased owing to a decrease in outer diameter (dynamic load radius of the tire) as compared to a tire of normal internal pressure. In a method for detecting decrease in internal pressure on the basis of a relative difference in rotational angular velocities of tires,                DEL={(F1+F4)/2−(F2+F3)/2}/{(F1+F2+F3+F4)/4}×100(%)is employed as a judged value DEL (reference should be made to Japanese Unexamined Patent Publication No. 305011/1988). Here, F1 to F4 denote rotational angular velocities of a front left tire FL, a front right tire FR, a rear left tire RL and a rear right tire RR, respectively.        
Since tires are manufactured to include variations (initial differences) within standards, effective rolling radii of the respective tires (a value obtained by dividing a distance which has been traveled by a single rotation by 2π) are not necessarily identical even though all of the tires are at normal internal pressure. This will result in variations in the rotational angular velocities Fi of the respective tires, and when the above DEL values are calculated in this state, it might happen that DEL=0 is not satisfied even if the internal pressure is normal to issue erroneous alarm.
Processes of obtaining correction coefficients for correcting variations caused during the manufacture of tires (initialization) are therefore performed. More particularly, the following three coefficients are obtained.                F2=fac1×F1        F3=fac3×F1        F4=fac3×fac2×F1        
Here, fac1, fac2 and fac3 are correction coefficients which are obtained at normal internal pressure, wherein fac1 is a correction coefficient for correcting differences in effective rolling radii owing to initial differences between right and left front tires FL, FR; fac2 is a correction coefficient for correcting differences in effective rolling radii owing to initial differences between right and left rear tires RL, RR; and fac3 is a correction coefficient for correcting differences in effective rolling radii owing to initial differences between an average of the front axial tires ((FL+FR)/2) and an average of the rear axial tires ((RL+RR)/2), and these correction coefficients fac1, fac2 and fac3 are obtained through running tests when the tires are at normal internal pressure.
From among these correction coefficients, fac3 represents a front and rear wheel ratio, and is a value necessary for accurately obtaining a DEL value.
Since the front and rear wheel ratio is obtained on the basis of wheel speeds of the four wheels, such a value is affected not only by the variations caused during the manufacture of tires but also by slip caused through the driving. Since it is accordingly difficult to obtain effective front and rear wheel ratios when the vehicle is running on a flat road at constant speed, it is preferable that such ratios are obtained only when the vehicle is in a gradually decelerating condition.
However, a condition in which “the vehicle is in a gradually decelerating condition” might be extremely rare depending on running conditions. In such a case, effective front and rear wheel ratios can be hardly obtained, and it might take a long time until front and rear wheel ratios necessary for judging decompression are obtained.